the transparency of the Universe and the transparency of the university

The highlight of my day was a wide-ranging conversation with Suroor Gandhi (NYU) about cosmology, career, and the world. She made a beautiful connection between a part of our conversation in which we were discussing the transparency of the Universe, and new ways to study that, and a part in which we were discussing the transparency with which the University speaks about disciplinary and rules cases, which (at NYU anyway) is not very good. Hence the title of this post. On transparency of the Universe, we discussed the fact that distant objects (quasars, say) do not appear blurry must put some limit on cosmic transparency. On transparency of the University, we discussed the question of how much do we care about the behavior of our institutions, and changing those behaviors. I'm a big believer in open science, open government, and open institutions.

I've been privileged these years to have some very thoughtful scientists in my world. Gandhi is one of them.

publication and collaboration policies

I spent some time in travel working on ideas for the Terra Hunting Experiment's publication, collaboration, and data-release policies. Megan Bedell (Flatiron) and I are not doing this in any official capacity; we are just brainstorming things that might be a good idea. One theme of our comments is that we want to make sure that the rules very strongly incentivize participation in the project by postdocs and students, who often don't have long enough time horizons to be at one institution for the full scientific arc of a project in this space. Another theme of our ideas is transparency: The Sloan Digital Sky Survey rules do a lot with transparency, and it works well there. When things are transparent to all, you often need fewer rules, because transparency leads to constructive, inclusive discussions.

talks all day; Dr Gorbenko

Roberto Sanchis-Ojeda (Berkeley) arrived for a few days of hacking. He is responsible for finding some very short-period transiting exoplanets. We didn't get a chance to set our goals for the day, because the day was packed with talks:

Victor Gorbenko (NYU) gave an absolutely excellent PhD defense. Congratulations Dr Gorbenko. Among other things, he was considering the difference between the theoretical (analytic) action of massless modes on the 1+1 worldsheet of a string (not a 0+1 wordline but s 1+1 worldsheet) and a lattice calculation. In this comparison, he found massive modes. He very nicely connected this work to larger questions in string theory, but also to the origins of string theory in QCD.

At lunch, Guido D'Amico (NYU) spoke about axions, axion–photon interactions, and cosmic transparency. This is related to work I did with Bovy and with More in the optical. He advertised some clever ideas he has about about using the Planck data to constrain this sector using millimeter wavelengths. He has been implementing some of these ideas at #astrohackny.

At the end of the day, Neil Lawrence (Sheffield) spoke about deep machine-learning models built from nested Gaussian Processes. The methods show a lot of promise and connect well to things we are thinking about in CampHogg.

dust extinction and reddening

I worked out the details (on paper) of two projects we are thinking about relating to dust attenuation. In the first, we look at stellar spectra in the SDSS as a function of Galactic reddening and determine the dust attenuation law at high resolution. In the second, we look at quasar photometry in the ultraviolet as a function of line-of-sight separation from galaxies of differing properties to get the dust–galaxy cross-correlation function.

quasar absorption lines, submitted

Nikhil Padmanabhan (Berkeley) showed up at NYU today, and we discussed various matters related to BOSS and the baryon acoustic feature. Nikhil argued that we do not yet have a fully worked-out plan for analyzing the quasar absorption-line spectra, and measuring the correlation function therefrom. We discussed this in detail and I got a bit interested in the data analysis problem, which, when written down correctly, is quite difficult: It involves marginalizing over all hypotheses about each spectrum's unabsorbed continuum.

In other news, Surhud More submitted the transparency paper to ApJ. Should be on arXiv on Monday.

transparency scoop, velocities, testing GR

More and Bovy found papers today that—to some extent—scoop our result on the transparency of the Universe with baryon acoustic feature and type Ia supernovae. We did some re-tooling to emphasize the new aspects of our work in this context.

Bovy and I discussed at length various issues related to the determination of the velocity field from Hipparcos. Bovy has implemented a beautiful system that determines the error-deconvolved distribution of velocities, even in the face of the issue that each data point has a different error. We discussed using his fit to the velocity field to predict the results of radial velocity surveys.

Bhuvnesh Jain (Penn) gave a great astro seminar on testing gravity using the comparison of lensing and kinematic / dynamical measures of the potential. He featured Adam Bolton's strong lenses as among the best tests of this, although Bolton's test is at smaller length scales (kpc) than the scales of interest to most cosmologists (Mpc to Gpc). Jain made a very good argument that if you want to test GR, you have to work at all scales and with all techniques; the different cosmological tests can only be combined or ranked if you believe GR.

insane theories, super-k-means

I can't say I did much research today, but while I failed to do research, Bovy (who is also attending a scientific meeting) looked at contemporary models that violate transparency to fix the supernovae Ia results in an Einstein—de Sitter Universe. These models are somewhat crazy, because they end up building epicycles to fix a problem that isn't really a problem, but in principle we will rule them all out with BOSS.

In my sliver of research time (and with Roweis's help), I figured out that PCA, k-means, mixture-of-gaussians EM, the analysis we did in our insane local standard of rest paper, and taking a weighted mean are all different limits of one uber-problem that consists of fitting a distribution function to data with (possibly) finite individual-data-point error distributions. I am trying to write something up about this.

Tolman and Etherington

In working on the More paper, I found myself looking through cosmography literature from 1929 through 1933. There is a series of papers by Tolman, in which he works out the Tolman test for the expansion of the Universe, which I think of as being a test of transparency and Lorentz invariance. Tolman worked out the test in the context of one world model (de Sitter's); his interest was in understanding the possible physics underlying the steady-state model; Etherington generalized it to a wider range of world models in 1933. After Etherington's generalization, the community should have realized that the test doesn't really test expansion per se, but it does test relativity and electromagnetism in that context.

tranparency: the monopole term

Worked on the paper I am writing with More on the transparency of the Universe, showing that the consistency of baryon acoustic feature (not oscillation) measurements with supernovae type Ia measurements provides a non-trivial constraint on Lorentz invariance and transparency. Right now this is not super precise, but it is highly complementary to measurements of absorption (presumably by dust) in lines of sight near galaxies, because there is no model-independent way to integrate the absorption signal correlated with galaxies to the mean, global value—what I would call the monopole term.

writing

I finished everything I could do on my project on the monopole term in the transparency with More; that project will now wait until he returns. I worked on writing up a principled approach to a side project that Lang and I have been discussing on finding variable sources in pure photon time streams.

CBR transparency

I tried to work out the limit on cosmic transparency implied by the accuracy of the COBE FIRAS experiment measurement of the blackbody spectrum. It provides an effective "Tolman test" because the experiment measures not just the spectral shape, but also the absolute amplitude of the intensity field.

deconvolution, cold streams, marginalization

In separate conversations, Lang and I and Marshall and I have been talking about image modeling and deconvolution. I finally realized today something very fundamental about deconvolution: A deconvolved image will never have a well-defined point-spread function, because the high signal-to-noise parts of the image (think bright stars in astronomy) will deconvolve well, while the low signal-to-noise parts (think faint stars) won't. So the effective point-spread function will depend on the brightness of the source.

Properly done, image modeling or deconvolution—in some sense—maps the information in the image, it doesn't really make a higher resolution image in the sense that astronomers usually use the word resolution.

This all gets back to the point that you shouldn't really do anything complicated like deconvolution unless you have really figured out that for your very specific science goals, it is the only or best way to achieve them. For most tasks, deconvolution per se is not the best thing to be doing. Kind of like PCA, which I have been complaining about recently.

In other news, Kathryn Johnston (Columbia) agreed with me that my first baby steps on cold streams—very cold streams—is probably novel, though my literature search is not yet complete.

In yet other news, Surhud More and I figured out a correct (well, I really mean justifiable) Bayesian strategy on constraining the cosmic transparency in the visible, by marginalizing over world models.

submission and resubmission

Bovy, with some help from Moustakas and me, got ready the galaxy-cluster transparency paper for resubmission in response to referee. The referee really made a big difference to the paper, because he or she recommended averaging the samples in a better way, which improved the results. I, with help from Barron and Roweis, got ready the Blind Date paper (on estimating image dates using proper motions) for resubmission. And I, with help from Lang, have promised my co-authors I will get the paper on faint-source proper motions ready for submission by the end of the week. That end approaches fast.

bimodality, transparency

I switched my search for low kurtosis directions in spectrum space into a search for bimodal directions. That is, I wrote down a scalar (which has to do with k-means with k=2) that decreases as a distribution becomes more bimodal. Then I searched Tsalmantza's high-variance PCA components for directions in the space that are most bimodal. I find three, perpendicular bimodal directions! Of course each one is a different version of the red–blue galaxy bimodality, of which I have been an unheard critic. More on this as I understand it better.

Surhud More (MPIA) and I began working this week on the monopole term in the opacity of the Universe, using the consistency of baryon-acoustic and supernovae measures of the expansion history to check the phase-space conservation of photons. This test is (nearly) independent of world model, as it depends almost entirely on purely special-relativistic considerations. We hope to have an LPU (least publishable unit) on the subject soon.

mean DB WD image, transparency

The image below shows a measurement of the cross-correlation between DB white dwarfs taken from the SDSS White Dwarf Catalog and far-ultraviolet photons observed by GALEX.

The central source in the image is effectively the average (mean) image of a DB white dwarf in GALEX; its morphology is (roughly) consistent with a GALEX point-spread function. The other sources in the image are residuals created by random bright FUV sources nearby to individual DBs; there weren't enough DBs to make this all average away; none of the nearby sources seen in the image are significant if you ratio this image to the root-variance image obtained by jackknife resampling.

Schiminovich and I spent a long time today discussing the possible effects that might make the same kind of average image not look like a point-spread function when the central sources are at cosmological distances, because of scattering and correlated sources. We also discussed how measurements of the average image as a function of sky position and redshift could constrain transparency.

transparency

Brice Ménard (CITA) gave a nice seminar at Columbia about angular correlations between background (redshifts one and higher) quasars and foreground (redshifts one third) galaxies. The correlations are dominated by lensing, but also have a small color term which is consistent with absorption by dust. His results on dust compare favorably to my results with Jo Bovy and John Moustakas, although he worked entirely in angular units; it is somewhat easier to interpret and model in projected transverse distance units (a simple modification to their current strategies). He had not done any de-projection or halo modeling of the results, so he couldn't precisely say what dust is associated with galaxies of each individual type.

After the talk Lam Hui (Columbia) and I discussed various matters transparent, including tests for the monopole (unclustered) term in attenuation, and possible other explanations for chromatic effects in photon propagation. For example, if the dark matter is made of substantial millilensing or microlensing lumps, and if quasars have a wavelength–size relation, there might be chromatic effects in the angular correlations.

transparency

I spent some enjoyable time working through Bovy's write-up of his work on the transparency or dust content of galaxy clusters (he is comparing mean spectra of red galaxies behind and not behind clusters taken from Berlind's group catalog in SDSS. As expected, we have very sensitive limits on the dust content of the clusters, and it also appears that we have a positive result. But the big challenge is to compute realistic error bars; most bootstrap or jackknife methods will underestimate them.

dust

At lunch I described Bovy, Moustakas, and my tentative detection of dust absorption in galaxy clusters using background luminous red galaxies. In the afternoon, Barron and I discussed how to flexibly include dust in our three-dimensional models of galaxies.

photon interactions with the dark sector

I attended a very nice seminar today by Aaron Chou (NYU) about his work on an experiment looking for oscillations between photons and dark particles like axions or equivalent. He has a null result, which rules out some experimental claims. I was interested in the work because it might be superseded by astronomical measurements of transparency, one of which I am in the process of making right now.

transparency of galaxy clusters

Moustakas and I sat down with Jo Bovy (NYU) to discuss his short-term project of measuring the transparency of galaxy clusters. We are interested in two things: (1) can we measure the amount of dust in the clusters, and does it make sense given the plasma temperature and the metallicity, and (2) can we rule out a class of naive axion models for the dark matter, which generically cause wavelength-dependent transparency variations in regions of high magnetic field? This all might sound crazy, but there hasn't been much work on the transparency of the Universe since the 80s, and the data have gotten much better. I expect us to put very strong limits and rule out some theories.

Or win the Nobel Prize!